This invention is directed generally to control circuits for hydraulic systems and more particularly to a novel control circuit for a hydraulic system which operates a snowplow blade.
Hydraulic operating systems for vehicle-mounted snowplow blades are well known in the art. Generally, such systems comprise hydraulic pistons mounted on a snowplow blade-carrying framework for raising and lowering the snowplow blade and for angling or turning the snowplow blade left and right. These hydraulic pistons are operated by a hydraulic pump in cooperation with a number of control valves for delivering hydraulic fluid to the respective pistons for achieving the desired motion left or right and up or down. This hydraulic pump may in turn be driven by a suitable vehicle power take-off or by an electric motor.
In order to be electrically operated by a suitable control circuit, the fluid delivery control valves as well as control valves for a hydraulic power take-off, if utilized, are of the electrical solenoid-operated type. Moreover, a suitable control circuit must be provided to energize solenoid valves and an electric motor, in installations where an electric motor is utilized to drive the hydraulic pump. In either case, the control circuit operates in response to operation of a suitable manual control by the vehicle operator.
While left and right angling or turning of the snowplow blade is a relatively simple operation, additional considerations arise in the raising and lowering or up and down motion of such a snowplow blade. For example, when the vehicle is to be driven on a street or highway, the controls for the raising of the blade must reliably hold the blade in the up position at all times. On the other hand, when in use for plowing, the blade must not be rigidly held in a given position but must "float" in order to follow the contour of the surface being plowed.
The existing hydraulic solenoid valve controls commonly make use of a normally closed valve for the down or floating position of the blade. Accordingly, this valve must be constantly energized when the blade is in the down or plowing position. However, if the vehicle should be stopped for a prolonged period or left overnight with the blade in the down position, considerable power drainage from the vehicle battery will result. The prior art has not heretofore offered an adequate solution for this problem.
Moreover, it is desirable that the manual control device provided in the vehicle cab be relatively simple to operate so that the operator may devote the majority of his time to the control and operation of the vehicle.